The properties of ceramics change greatly according to its manufacture process. It is thus indispensable to evaluate the properties of each ceramic product after the manufacture of the product. Conventionally, numerous methods for evaluating the properties of ceramics in either a destructive manner or nondestructive manner have been considered. Silicon nitride (Si.sub.3 N.sub.4) will now be described as one example of ceramics.
Silicon nitride is noted as an industrial ceramic used for making structural members such as automobile engine components, intended to work at high temperatures such as automobile engine components. Since sintering of silicon nitride alone is difficult, however, a sintering aid made of a low-melting point compound is generally added to silicon nitride powder for sintering. Oxides of aluminum, magnesium, yttrium, lanthanum, cerium, beryllium or zirconium are used, for example, alone or in combination as a sintering aid for sintering silicon nitride.
A sintered body of silicon nitride having a higher density can be achieved by use of the sintering aid than without such an aid. However, the sintering aid remains as a glass phase or a crystal phase of lower strength, hereinafter referred to as "glass phase or the like", in a grain boundary of a resultant sintered body. It is thus difficult to obtain a silicon nitride sintered body having the required mechanical property such as a desired strength. However, when the silicon nitride sintered body is manufactured by pressure sintering such as a Hot Press method or an HIP method, it is possible to reduce or eliminate the amount of the sintering aid previously added. Nevertheless, the manufacture of the silicon nitride sintered body by such pressure sintering has the disadvantage that it is difficult to carry out successive sintering steps, resulting in an increase in the manufacturing cost.
Accordingly, at present, a silicon nitride sintered body which is obtained industrially includes a glass phase or the like in its grain boundary. It is considered in general that mechanical properties such as the material strength in ceramics, such as silicon nitride, deteriorate with an increase in the amount of the glass phase or the like included in the ceramics. It is extremely significant to precisely measure the amount of the glass phase or the like included in the ceramics, in order to manufacture ceramics such as silicon nitride having excellent mechanical properties such as material strength. However, a nondestructive inspection which can be introduced into a production line of ceramics so that products travelling along the line can be directly inspected for measuring the amount of glass phase or the like, has been unknown heretofore.
More specifically, conventional methods of measuring the amount of glass phase or the like included in ceramics such as silicon nitride involve the following: A structure observation method permits observing the structure or texture of a sintered body having abraded surfaces by employing a microscope or the like; an X-ray diffraction method for photographing an X-ray diffraction image of a sintered body which is ground to have a predetermined thickness; a method for processing a sintered body in a predetermined shape and measuring an internal friction as disclosed in Japanese Patent Laying-Open No. 63-1955. In accordance with the foregoing conventional methods, it is impossible to measure the amount of glass phase or the like included in ceramics, in a nondestructive manner, and hence, only a sampling inspection can be carried out on the production line. Thus, there has been no practical possibility of guaranteeing the amount of the glass phase or the like in conventionally produced ceramic products.